Ponding water on flat roof causing problems

What Causes Ponding Water On Flat Roof?

When people see a pool of water settling on their roof they laugh it off, thinking it’s just normal and harmless. This shouldn’t be the normal reaction, as water ponding can have serious consequences. 

Don’t make the mistake of writing off the potential damage that’s being caused here. No matter how durable or new your roof is, just letting water sit on it can cause problems in the long term if not handled.

Thankfully, ponding water on flat roof doesn’t mean the end of the world- there are a couple of solutions available to fix your problem. (Hint: One of the best available solutions is silicone coating, or waterproofing your roof.)

What Is Water Ponding?

Put very simply, any significant amount of water that pools up on your roof for more than 24 hours can be considered water ponding. 

What Causes Water Ponding?

  • Poorly designed roof with no drain slope
  • Caved in structure over years of wear and tear
  • Caved in structure caused by heavy A/C units
  • Uneven areas on your flat roof are caused by either faulty installation or slight changes in roof slopes. 

Why Is Water Ponding Bad?

Water that’s stuck, means water that tries to find its way into your building. It’ll wear down roofing material, lead to mold, lichen, and algae growth, and is generally destructive- eventually leading to leaks. 

It is important to note that water may reside on your roof for 12-48 hours after a storm or frequent rain. Regardless, if you see any water ponding on your roof we recommend that you call a specialist to determine if your roof is at risk of damages. Lastly, water ponding can happen for countless reasons, but the best defense against any damages is to pick up the phone and be proactive. 

Ponding Water On Flat Roof? Here’s why it really happens.

Flat Roofs shouldn’t be flat. 

Water ponding happens when water settles on your roof for an extended time leaving your roof at increased risk of damages. This can happen for any number of reasons. 

The first major cause is created by the fact that our “flat” roofs aren’t flat. Architects or contractors concave roofs so that water naturally gravitates towards drains. Otherwise, the possibility for water ponding and leakage increases drastically. The issue that occurs with these roofs is that sometimes that downward slope is too great or uneven, which may lead to drain clogging or other issues taking place. 

The National Roofing Contractors Association, The American Institute of Architects, and the International Institute of Building Enclosure Consultants tell us that any roof that caves in more than 0.25’’ per foot are in danger of water ponding.

Roof Age

Age is one of the leading causes of water ponding. Unlike the first cause mentioned, age cannot be avoided. When a building ages the roof and foundations are likely to shift slightly and erode. This is called sagging. 

When your building succumbs to the limits of its longevity and starts to lean; this causes water to accumulate in a corner of the roof and potentially cause damage.

This is pretty common, especially with commercial buildings that have large A/C units. These units are heavy as hell, and that amount of localized dead weight on a roof is going to eventually cause some caving. 

Here’s a perfect example of that: 

Ponding water on flat roof

See how all of that water has ponded around the A/C unit? That’s extremely typical, and that’s why one of the first areas on a roof that we’ll inspect is around A/C units. 

Sometimes, depending on the extent of the ponding, the age of the roof, and whether or not the roof has any leaks already, we’ll recommend that commercial building owners apply some kind of waterproofing around A/C units, right from the beginning. 

 Lack of Proper Drainage

In a previous section, we briefly mentioned problems related to the clogging of drains or related issues. Well, these problems occur more than you might think. On countless occasions, we have witnessed improperly placed drains, or just a lack of drains to prevent ponding. It doesn’t matter if you see gutters or drains on your roof, if the roof isn’t sloped in a way that leads water towards the drains, then water isn’t going to drain. A drainage issue can lead to water ponding and can cause severe damages to your property. If you’re interested in finding out more about drainage systems for commercial buildings, check out this article.

There can be countless causes of water ponding, but these three are the most common that we have seen in the past. But regardless of the cause of your water ponding, we urge everyone to schedule an immediate appointment with a specialist or contractor to properly diagnose your ponding. Please be proactive and prevent any damage to your property. 

Risks of water ponding

First of all, water ponding on any roof is a very serious problem. It can lead to serious damages to your property. More specifically, it increases the chances of the growth of organisms, further sagging of a roof, and leaking.

Growth of Organisms

As we all know, when we leave a large body of water unattended it can introduce a few uninvited guests into the equation. An increase in algae, vegetation, and insect population are all negative, possible consequences of water pooling when untouched. The introduction of things to your building can have extremely negative repercussions. The vegetation can make its way into your insolation, wood, metal framing, or concrete and cause serious damages. Moreover, these plants can also introduce moisture into your structure causing it to rust and decay at an accelerated rate. 

Roof Sagging

When water accumulates on your roof for extreme periods it can cause damage to the out layer of asphalt, concrete, or any material used to construct your roof. With enough time and the right conditions, your roof will continue to cave inwards. If left completely neglected the consequences can be deadly. Your roof will continue to sink and is at risk of caving inwards. 


If you ignore the ponding water and constantly allow it to just sit on your roof- eventually you’re going to have some major issues. The water can weather away from the outer layer of roofing. Then, it can seep through small cracks in the roof into the frame, insulation, and rooms of the building. 

How to fix water ponding

Water ponding is serious and can cause problems to the structural integrity and longevity of your roof. So, it would only be natural for you to be looking for ways to avoid any extensive damages. Well, the first thing you can do is get your roof done right the first time around. Hire a trusted contractor or specialist to properly slope and drain your roof. But, if you are beyond this stage and are looking for solutions. Look no further.

Clogged Drains?

Well, the first thing you can try doing to alleviate the problem is to check if your drains are clogged. If you have drains that are clogged they are heavily contributing to the water pounding on your roof. So, be sure to check if your drains are clogged.

Tapered Insulation

Tapered insulations are one of the most frequently used solutions to water ponding. Tapered insulation or tapered polyisocyanurate is a form of flat roof insulation with a built-in slope. What this does is add another layer of security to your roof, while implementing a fresh system for running any water off your roof. It is important to note that the tapered insulation is held to the same standards as any other sloped roof.

The National Roofing Contractors Association, The American Institute of Architects, and the International Institute of Building Enclosure Consultants tell us that any roof that caves in more than 0.25’’ per foot are in danger of water ponding.

Silicone Coating Waterproofing

Another excellent method to dealing with ponding water is to have a waterproofing silicone coating installed. Silicone coatings are an excellent, cost-conscious option for dealing with ponding water. 

Workers on a roof applying silicone waterproofing coatings

Plus, a properly installed coating of silicone will essentially renew your roof, adding up to 15 years to its lifespan. Not to mention, silicone is hydrophobic, meaning it rejects water, so you’re not going to have any problems with leaks, or any damage to your roof. Topping it all off, the white coating on your roof is going to dramatically impact cooling costs, and depending on where you are, can reduce cooling costs by up to 30%. 

Your Drains aren’t doing their job.

In the situation that your roof does not have a drainage system then, call a specialist in order to immediately alleviate the issue. Drains are the single best defense against water ponding that exists. 

Unfortunately, if your roof wasn’t built properly and sloped as it should have been, drains are not going to be a viable solution. However, that’s something that a roofing professional should be able to quickly determine. 

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